README.md is manually assembled from several docs/ files with transformations (header demotion, link absolutization, TIP callout wrapping). This was only validated by a test, not enforced at commit time.

Adds a Jinja2 template + pre-commit hook that auto-generates README.md from doc sources, replacing the manual sync workflow.

• docs/README.md.jinja — Template with {{ include_doc(...) }} calls that pull in content from docs/ with per-include transformations
• hooks/fix-readme.py — Fixer hook (exit 1 on modification, following existing export hook pattern). _include_doc() supports skip_lines, demote_headers, replacements dict, and tip_text callout wrapping
• .pre-commit-config.yaml — Registers fix-readme at priority 1 (after markdownlint/prettier process doc files at priority 0)
• Removed tests/docs/test_readme.py — The test_assemble_readme_from_docs test is now redundant since the hook enforces the same invariant at commit time
• Updated .github/workflows/ci.yml — Added docs/** and **/*.md to paths-ignore (for both push and pull_request), matching the patterns already used by codecov, codspeed, codeql, and zizmor workflows. The static-checks.yml workflow intentionally has no paths-ignore and continues to run on all changes.

Template usage in docs/README.md.jinja:

{{ include_doc("docs/index.md", skip_lines=2, demote_headers=false) }}

{{ include_doc("docs/cli/overview.md", replacements={"](reference.md#": "](https://usethis.readthedocs.io/en/stable/cli/reference#"}) }}

{{ include_doc("docs/about/similar-projects.md", replacements={...}, tip_text="You can still use usethis") }}

Jinja2 is already a dev dependency.